The present invention relates methods of treating subterranean formations. More particularly, the present invention relates to methods for using derivatized gelling agents in subterranean treatment fluids.
Treatment fluids are used in a variety of operations and treatments performed in oil and gas wells. Such operations and treatments include, but are not limited to, production stimulation operations, such as fracturing, and well completion operations, such as gravel packing and frac packing.
An example of a production stimulation operation using a treatment fluid is hydraulic fracturing. That is, a type of treatment fluid, referred to in the art as a fracturing fluid, is pumped through a well bore into a subterranean zone to be stimulated at a rate and pressure such that fractures are formed or enhanced in a desired subterranean zone. The fracturing fluid is generally a gel, emulsion, or foam that may comprise a particulate material often referred to as proppant. When used, proppant is deposited in the fracture and functions, inter alia, to hold the fracture open while maintaining conductive channels through which such produced fluids can flow upon completion of the fracturing treatment and release of the attendant hydraulic pressure.
An example of a well completion operation using a treatment fluid is gravel packing. Gravel packing treatments are used, inter alia, to reduce the migration of unconsolidated formation particulates into the well bore. In gravel packing operations, particulates, referred to in the art as gravel, are carried to a well bore in a subterranean producing zone by a servicing fluid known as a carrier fluid. That is, the particulates are suspended in a carrier fluid, which may be viscosified, and the carrier fluid is pumped into a well bore in which the gravel pack is to be placed. As the particulates are placed in the zone, the carrier fluid leaks off into the subterranean zone and/or is returned to the surface. The resultant gravel pack acts as a filter to separate formation solids from produced fluids while permitting the produced fluids to flow into and through the well bore. While screenless gravel packing operations are becoming more common, traditional gravel pack operations involve placing a gravel pack screen in the well bore and packing the surrounding annulus between the screen and the well bore with gravel designed to prevent the passage of formation particulates through the pack with produced fluids, wherein the well bore may be oriented from vertical to horizontal and may extend from hundreds to thousands of feet. When installing the gravel pack, the gravel is carried to the formation in the form of a slurry by mixing the gravel with a viscosified treatment fluid. Such gravel packs may be used to stabilize a formation while causing minimal impairment to well productivity. The gravel, inter alia, acts to prevent the particulates from occluding the screen or migrating with the produced fluids, and the screen, inter alia, acts to prevent the gravel from entering the well bore.
In some situations the processes of hydraulic fracturing and gravel packing are combined into a single treatment to provide a stimulated production and an annular gravel pack to prevent formation sand production. Such treatments are often referred to as “frac pack” operations. In some cases the treatments are completed with a gravel pack screen assembly in place with the hydraulic fracturing treatment being pumped through the annular space between the casing and screen. In this situation the hydraulic fracturing treatment ends in a screen out condition creating an annular gravel pack between the screen and casing. This allows both the hydraulic fracturing treatment and gravel pack to be placed in a single operation. In other cases the fracturing treatment may be performed prior to installing the screen and placing a gravel pack.
A variety of methods are used to create the treatment fluids typically used in subterranean operations. Often, a polysaccharide or synthetic polymer gelling agent is used to impart viscosity to an aqueous treatment fluid to, among other things, enhance proppant transport and reduce fluid loss from the treatment fluid into the formation. Frequently, a crosslinking agent, such as a metallic compound, is added to further enhance the viscosity of the fluid by coupling, or “crosslinking,” the polymer gelling agent.
One common gelling agent used in treatment fluids is guar, a galactomannan type of polysaccharide which may be crosslinked to yield a high gel strength for suspension, and yet they can be relatively easily broken when desired (that is, the viscosity of the treatment fluid viscosified with guar may be reduced when desired). Because of its abundance, price, and geometry favorable to crosslinking, guar is the most commonly used gelling agent in treatment fluids.
Derivatized gelling agents may also be used. Derivatized polysaccharides presently used in subterranean formations are generally formed by placing the polysaccharide (generally a guar) in a high pH environment (generally at a pH of about 11 or greater) and exposing the polysaccharide to a reactive group having the desired derivatizing moiety. Some suitable reactive groups are halides and epoxides. Chloroacetic acid, sodium chloroacetate, ethylene oxide, and propylene oxide are the most commonly used derivatizing agents. It would be highly desirable in the industry to have a method of creating a derivatized polysaccharide suitable for use in a subterranean application that could be made without resorting to such harsh reaction conditions and without the need for extra process steps.